The prior art pertinent to the instant invention is discussed at length in U.S. Pat. No. 4,056,465 issued to one of the present inventors. The present invention is directed to improvements over the system disclosed in said patent.
The selective production of biomass species capable of removing phosphate values and producing a non-bulking sludge of rapid settling characteristics is attained in accordance with the above cited patent by strictly maintaining specific conditions in an initial operating zone where incoming wastewater and recycled sludge from secondary clarification are mixed to form a mixed liquor. Under these conditions the proliferation of undesired high surface area microorganisms is avoided, while substantial quantities of Biological Oxygen Demand (BOD) are sorbed from the influent wastewater by organisms having the capability of doing so under these conditions.
The conditions employed in this initial zone are termed "anaerobic" in the above mentioned patent. More specifically, it is required that the amount of oxygen and/or oxidizing agents introduced into this zone be minimized so that the biomass is forced or stressed by the conditions to hydrolyze polyphosphates contained within the walls of the cells in order to provide the energy necessary to sorb BOD. If any significant amount of oxygen or oxidizing agents is present in this zone, the oxygen (rather than the contained polyphosphate) will be the source of energy for the sorption of BOD to the detriment of the desired mechanism of this invention. Accordingly, efforts must be made to minimize, if not totally exclude, the presence of oxygen in this zone. Positive steps which can be taken include covering the zone to prevent contact with atmospheric oxygen, blanketing the zone with nitrogen, bubbling nitrogen through the mixed liquor to strip any oxygen that might have been entrained, etc. Obviously, no positive action is permitted which might introduce oxygen, such as, aeration by use of a surface aerator or introducing an oxygen containing gas, e.g. air, via a sparger or diffuser. No oxygen is to be deliberately introduced into this zone and this zone is to be maintained substantially free of oxygen and oxidizing agents. In any commercial plant it is almost impossible to ensure complete absence of all oxygen or oxidizing agents and the presence of a minor, insignificant amount of oxygen, while never desirable, can be tolerated without destroying the operability of this process. Accordingly, when no deliberate introduction of oxygen is being practiced, e.g. aeration, isolated points of detectable dissolved oxygen (DO) of up to 0.7 ppm, but preferably less than 0.3 ppm, can be acceptable, at least for short periods of time. Any extensive or high presence of oxygen or any significant level of continuous oxygen introduction is, of course, unacceptable and contrary to the requirements of this process.
Subsequently, the mixed liquor treated in the initial sorption zone is then treated in an oxidation zone where the food initially sorbed in the sorption zone is oxidized and any remaining BOD is sorbed and oxidized. During this oxidation stage the energy previously lost by hydrolysis of polyphosphates is recouped and polyphosphates are reformed and stored within the aerated biomass, thus removing phosphate from the mixed liquor.
If denitrification of the wastewater is also desired, the patent indicates that an anoxic zone may be interposed between the anaerobic and the oxidation zone.
The term "anoxic" is defined in the aforesaid patent "as the condition existing within a sewage treating zone wherein BOD is metabolized by nitrates and/or nitrites in initial total concentrations higher than about 0.5 ppm expressed as nitrogen, and dissolved oxygen is less than 0.7 ppm, preferably at less than 0.4 ppm".
As further described in the patent, in order to assure adequate oxygen presence in the oxidation zone to effect desired metabolism of BOD and the desired phosphate uptake, the dissolved oxygen content (DO) of that zone should be maintained above 1 ppm and preferably above 2 ppm. In the several operating examples of the patent the average DO employed in the total oxidation zone is close to or above 6 ppm.
U.S. Pat. No. 4,162,153 which involves improvements in the wastewater treating system described above, also teaches that this type of system must be operated at a DO concentration of at least 1 ppm in the oxidation zone. Nowhere in any of these references is it indicated that this type of system can function effectively at substantially lower D.O. levels.
Systems of the type disclosed in FIG. 1 of U.S. Pat. No. 4,056,465 having an initial sorption zone followed by an oxidation zone are sometimes denominated "A/O".RTM.systems. Systems of the type illustrated in FIG. 2 of said patent, having an anoxic zone intermediate the sorption and oxidation zones are referred to as "A/A/O" or "A.sup.2 /O".TM.systems.